Aging reduces the number of mesenchymal stem cells (MSCs) in the bone marrow which leads to impairment of osteogenesis and subsequent bone loss (osteoporosis). Previous attempts to use systematic MSC transplantation have failed to regenerate bone due to inability of the transplanted MSCs to home to bone. MSCs transplantation may be a therapeutic option to treat bone loss if the cells could be directed to bone and induced to differentiate into osteoblasts. Our research group has developed a novel method to direct transplanted MSCs to bone by creating a compound with high affinity to both the ?4?1 integrin on MSCs (a synthetic peptidomimetic ligand, LLP2A) and bone (alendronate). The resulting conjugate, LLP2A-Ale, significantly increased homing of the transplanted MSCs to bone and underwent osteoblast differentiation in a xenotransplantation model. LLP2A-Ale also increased bone formation and bone mass in young immune competent mice. We have shown that MSCs attach to bone and undergo osteogenic differentiation when they are directed to the bone surface. Although LLP2A-Ale is effective in young animals, the compound failed to significantly augment bone formation in aged animals unless it was used in combination with systemic MSC transplantation. Moreover, data on disease model is lacking. Therefore, we propose testing the effectiveness of LLP2A-Ale in directing the transplanted MSCs in augmenting bone formation in animal models of osteoporosis and fracture healing. We hypothesize that LLP2A-Ale will direct the transplanted MSCs to the bone surfaces and facilitate new bone formation in the treatments of severe osteoporosis caused by aging, hormone deficiency or other disorders. To test this hypothesis, we propose the following two aims. 1. To determine the efficacy of LLP2A-Ale, or in the combination with MSC transplantation, in the augmentation of bone formation in animal models of osteoporosis. 2. To determine if LLP2A-Ale alone or in combination with MSCs increases MSC engraftment and accelerates fracture healing. The goal of this project is to test whether we can build new bone as a novel treatment for osteoporosis by using LLP2A-Ale to direct transplanted MSCs to the bone. Our preliminary data suggests that LLP2A-Ale can in fact direct transplanted human MSCs to bone and undergo osteoblast differentiation. This is of enormous importance for using MSCs in the treatment of osteoporosis since the major roadblock has been inefficient homing of MSCs to bone. Our novel approach would be effective in patients with primary osteoporosis induced by estrogen deficiency or advanced aging, and in those with secondary osteoporosis due to medications such as glucocorticoids or chemotherapy. Our lead compound, LLP2-Ale, once tested thoroughly in preclinical studies, has great potential to be taken into clinical practice.